1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and a process cartridge.
2. Description of the Related Art
Researches and developments of electrophotography have been conducted with various inventive ideas and technical approaches. In electrophotography, an electrostatic latent image is formed by charging and exposing a surface of an electrostatic latent image bearing member. The electrostatic latent image is developed with a color toner to form a toner image. Then, the toner image is transferred onto a recording medium such as transfer paper, and fixed by, for example, a heat roller to thereby form an image. The toner remaining on the electrostatic latent image bearing member without being transferred is removed by, for example, a cleaning blade.
In recent years, color image forming apparatus using electrophotography have broadly been employed, and digitized images are easily available. Thus, it is desired to make an image to be printed at higher definition. Based on a study on higher resolution and gradation of an image, a spherical toner was developed in order to faithfully reproduce an electrostatic latent image. Also, the spherical toner has been researched to be further spheroidized and small-sized. Toners produced by the pulverizing methods have limitations in the above properties, i.e., sphericity and size. Therefore, so-called polymerized toners produced by a suspension polymerization method, an emulsion polymerization method or a dispersion polymerization method, which are capable of being spheroidized and small-sized, have been employed.
However, in the case of the polymerized toner, deterioration of cleanability due to its sphericity has been problematic.
In the electrophotographic image forming apparatus, a method has been widely used in which a toner image formed on an electrostatic latent image bearing member serving as an image bearing member is transferred onto a recording medium, followed by applying heat and pressure at a fixing step to thereby fix the toner image on the recording medium.
During fixing, the recording medium may be wound around a fixing roller, which is problematic. This problem has been solved by adding a releasing agent into a toner.
Although a large amount of the releasing agent present on a toner surface permits fixability to be improved, it has been problematic in that an electrostatic latent image bearing member may be contaminated due to migration of the releasing agent and an external additive to the electrostatic latent image bearing member.
That is, in the case of the spherical toner, a residual toner remaining on the electrostatic latent image bearing member is difficult to be removed. Additionally, when an unnecessarily large amount of the releasing agent is present on the toner surface, it has been problematic in that the residual toner remaining on the electrostatic latent image bearing member may easily cause image loss.
In recent years, a contact type one-component development has been employed in order to respond to a demand for small-sizing. In the contact type one-component developing system, a surface of an electrostatic latent image bearing member is in contact with a surface of a toner bearing member in a developing apparatus, and the toner bearing member rotates faster than the electrostatic latent image bearing member, which causes strong rubbing stress.
The toner is allowed to rub against the electrostatic latent image bearing member by the action of the rubbing stress. Therefore, the toner is strongly adhered to the electrostatic latent image bearing member, which deteriorates cleanability. Additionally, the electrostatic latent image bearing member is finely scratched, and then toner components are adhered to the scratched portions, which causes contamination of the electrostatic latent image bearing member. These are problematic.
As for a fixing system, there has been proposed to employ an oilless fixing system to thereby prevent high temperature offset due to a releasing agent and ensure releasability from paper. However, this proposition is problematic in that the releasing agent causes contamination of the electrostatic latent image bearing member.
To solve the above problems, there has been proposed to specify an amount of a releasing agent present on a toner surface to thereby reduce contamination of an electrostatic latent image bearing member (see Japanese Patent Application Laid-Open (JP-A) No. 2011-133518). However, in this proposition, a relatively large amount of the releasing agent is present on the toner surface, and the relationship between the electrostatic latent image bearing member and a toner bearing member is not taken into account. Therefore, when used in the contact type one-component developing, the proposition is unsatisfactory to prevent contamination of the electrostatic latent image bearing member.
There has been proposed to specify a property of a binder resin contained in a toner, and pressure between an electrostatic latent image bearing member and a toner bearing member to thereby reduce contamination of the electrostatic latent image bearing member (see JP-A No. 2002-214835). Also, there has been proposed to specify the type of a releasing agent used in a toner, and pressure between an electrostatic latent image bearing member and a toner bearing member to thereby reduce contamination of the electrostatic latent image bearing member (see JP-A No. 2003-035968).
However, in the above propositions, the amount of the releasing agent present on the toner surface, which is a cause of contamination of the electrostatic latent image bearing member, is not controlled, and measures are not taken to improve image quality. Therefore, this proposition is problematic in that the contamination of the electrostatic latent image bearing member is insufficiently prevented and high quality image cannot be obtained.
There has been proposed to specify the amount of a releasing agent present on a toner surface to thereby prevent the releasing agent from adhering to an electrostatic latent image bearing member (see JP-A No. 2003-207925). However, in the contact type one-component developing, the adhesion occurs greatly depending on contact pressure (developing pressure) between the electrostatic latent image bearing member and a toner bearing member. Therefore, this proposition is problematic in that the contamination of the electrostatic latent image bearing member is insufficiently prevented and high quality image cannot be obtained.